-, Indian Journal of Marine Sciences Vol. 28, December 1999, pp. 449-454 The depositional history of late Quaternary sediments around Mangalore, west coast of India B R Manjunatha Department of Marine Geology, Mangalore University, Mangalagangothri 574 199, Kamataka, India and K Balakrishna Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India Received 15 June 1998. revised 14 September 1999 Netravati and Gurpur are relatively large rivers drain the hinterland near Mangalore, form a common estuary before they debouching into the Arabian Sea. Lithologic successions observed in a number of bore holes and dug wells indicate that Netravati and Gurpur rivers drained into the Arabian Sea independently during the last glacial period when the sea level was about 100-138 m below the presenrlevel. In contrast to Netravati, the lower course of Gurpur has migrated southerly in four stages for a distance of 8 km and at presenC forms a common estuary with the Netravati river. This is because of drowning of the river channel due to rapid rise in sea level during the early Holocene and growth of barrier spit under the strong influence of southerly littoral currents during late Holocene when the sea level was relatively stabile. The rate of in- filling of alluvial and marshy sediments during late Pleistocene to early Holoc<tne (0.5-5.0 mmlyr and 0.33-4.33 mmlyr respectively) is about two-three times slower than that for barrier spit sands accumulation (0. I 4-1.14 mmlyr) during the late Holocene. West coast of India is characterized by both emergent and submergent features because of long term changes in sea level, climate, lithology, structure and local tectonic movements l . Nevertheless, submergent features like estuaries, marshes, tidal swamps, mud flat deposits, spits and bars, tombolos etc. dominate the coastline l - 4 • The Maharashtra coast is dominated by ria-type estuaries into which seawater intrudes up to 30-45 km inland and river mouths are widened into broad creeks 2 • 4 • Estuaries of the Karnataka coast are shallow, characterized by narrow river mouths and seawater intrusion can barely be traced beyond 15-20 km inland. The Kerala coast is characterized by estuaries and lagoons'. The Karnataka coastline is generally straight, trending NNE-SSW of the Precambrian (F2) folding 5 . The coastal tract can be demarcated into the northern Uttara Kannada and the southern Dakshina Kannada districts_ The former has a coastline of 150 km, covering an area of 8000 km 2 , whereas the latter has 140 km long coastline and an area of 8440 km 2 . The coastal zone is characterized by heavy rain fall (>3000 mmlyr) and generally high temperature (>30°C). The coastal tract is highly dissected and the landscape is the result of extensive Tertiary denudation because of effect of tropical climate 6 . The west coast of India, particularly Konkan, Karnataka and Kerala coasts, is relatively tectonically stable 7 . Therefore, sea level changes observed here are due to glacio- eustacy whose impact on Quaternary sedimentation processes can be best understood by the study of sediments in these coasts. The major part of the Karnataka coast comprises of sandy beaches 7 , which vary in width from a few meters to 200 m. Beaches of the Uttara Kannada district occur in between estuaries and rocky headlands, whereas that along Dakshina Kannada are almost straight, interrupted by a number of estuaries without headlands 8 . Therefore, the study of the Dakshina Kannada coast is ideal for understanding long-term depositional history of coastal sediments. The study area includes the Netravati-Gurpur estuary and the adjacent landmass (Fig. I). Netravati and Gurpur rivers originate in the Western Ghats, flow for 148 and 87 km respectively covering a total area of 4257 km 2 • These two rivers unite to form a common estuary before debouching into the Arabian Sea. However, observation of lithologic slIccession in a number of drilled boreholes and dug welis indicate that these rivers had independent confluence points with the Arabian Sea. The objectives of this paper are to: (I) identify reasons for shifting of the river course,